Offset lithographic printing has remained a most popular method of printing for many years. An important reason for this is the relative ease with which offset lithographic printing plates can be produced. Currently, the most widely-used method for plate preparation has remained that which utilizes specially-prepared masking films through which pre-sensitized printing blanks are selectively hardened or softened (according to the chemistry of the plate) by exposure to ultra-violet light. The plate then undergoes a development process, during which the more soluble regions of the plate (which may be the exposed or the unexposed areas) are washed away. A detailed description of the system and the plates used can be found in Chapter 20 of the book Printing Materials: Science and Technology by Bob Thompson, published by Pira (Leathershead, Sussex, UK), 1998.
In recent years, various considerations have arisen that point to advantages for modification of hitherto generally-accepted practices. With the advent of computers, information for printing is prepared digitally and it has become preferable to use this digital information as directly as possible in plate preparation. One obvious way would be to eliminate the masking film. Not only are these films a source of expense, but the most widely-used films are based on silver chemistry whereby the exposure and handling of the film must be in a light-excluding environment. In addition, the exposed film must be processed with chemical solutions which are unstable, messy and environmentally problematic.
One answer is to be found in computer-to-plate (CTP) systems whereby the offset lithographic plates are directly imaged with a light source which is modulated to correspond to the digital information from the computer. Thus the film intermediate is completely eliminated. It would have been easy and convenient if the UV sensitive pre-sensitized plates previously used for imaging with an intermediate film could have been used for direct digital imaging. However, it has been found necessary to either sensitize plates for imaging with visible light or to develop plates that are sensitive to radiation in the near infrared. The reason for this is that UV lasers are very expensive and difficult to modulate. Visible and infrared lasers are more readily available and less costly and easier to modulate to produce the digital signal necessary for imaging.
U.S. Pat. No. 4,486,529 describes the sensitizing of a negative working diazo system to the 450 nm to 530 nm region for imaging with, for example, argon-ion lasers with most of its energy being emitted in the 488 nm to 514.5 nm region. Such systems require a cover layer of, for example, polyvinyl alcohol to protect against oxygen inhibition and may also require a post-image heating stage. Sensitivity of diazo systems which are used in the coatings of pre-sensitized plates is generally in the region of 150 to 600 mJ/cm2. This type of plate, like most pre-sensitized plates, needs processing with a strong alkali, although attempts are being made to develop CTP systems that are processless. The subject of CTP systems can be found in Chapter 21 of the book (Thompson, 1998) cited above.
U.S. Pat. No. 5,339,737 Lewis et al describes the processless preparation of offset lithographic printing plates, wherein the upper layer or layers of the plate are ablated away. The upper layer is either oleophobic for waterless plates or hydrophilic for conventional wet-process plates. The substrate is oleophilic in both cases. U.S. Pat. No. 5,353,705 Lewis et al is similar to the previous patent, but describes additional layers for secondary partial ablation. U.S. Pat. No. 5,487,338 is similar, but includes reflective layers.
All of these inventions involve multi-layered plates which are expensive to produce. Also, for such multi-coated systems, it is more difficult to maintain a consistent standard of quality from plate to plate. So called processless plates that are imaged by laser ablation either involve a scrubbing stage to remove ablated debris from the surface, or they need extraction systems to remove debris ‘on the fly’ as it is being produced during imaging.
Recently, it has been found that there are ways of imaging UV-sensitive plates by digital means, permitting the development of machines for use in CTP systems. Examples of this are the UV-Setter® 710 and ProSetter®, CTPs produced by basysPrint GmbH of Boizenburg, Germany. These are flatbed image setters based on a Spatial Light Modulator device working in transmission and a non-coherent UV light source. U.S. patent application Ser. No. 09/312,763 assigned to Scitex Corporation, describes the invention of a digital image-setter utilizing a high-resolution micro-display which can be used inter alia to image pre-sensitized offset lithographic plates which are sensitive to either UV or visible light.
With the development of these technologies, there is a need for inexpensive plates highly-sensitive to either UV or visible light. It would also be beneficial to be able to process the plate without recourse to highly-alkaline solutions that, in many countries, is prohibited by law from being disposed of in sewage systems due to environmental hazards.
A reason for simplicity of processing can be found in the development of imaging-on-press. Although the concept of imaging-on-press has been described in, for example, U.S. Pat. No. 3,654,864 (Ovshinski), U.S. Pat. No. 3,741,118 (Carley), and U.S. Pat. No. 4,718,340 (Love III), printing presses incorporating this principle, such as the GTODI by Heidleberg, have only appeared within the past few years. Such presses utilize waterless plates imaged by infrared ablation where post-imaging processing is by a cleaning method to remove ablated material, rather than by image development. As the processing has to be on press, it has to be relatively simple.
Further developments in plate design have followed the path of elimination of a plate substrate and, in its stead, the press cylinder which traditionally holds the plate becomes a reusable lithographic ‘master’ surface. U.S. Pat. No. 4,718,340 (Love III) describes such a process which, in one embodiment, spreads an oleophilic material onto a hydrophilic surface and then removes oleophilic material from the non-image areas. As is stated in the Abstract of this prior-art patent, no photo-induced chemical reaction or latent imaging development steps are required at any time.
All present pre-sensitized UV sensitive offset printing plates have a common constraint. The manufactured plates are coated in continuous rolls in a factory where the plates are then cut to size, boxed and sent to the customer. The plate is removed from the box by the customer and placed on a machine for imaging. In the case of UV sensitive plates, it has been the general rule that they are placed in emulsion-to-emulsion contact with an imaged masking film, imaged by flood UV and then the film is peeled off before processing the plate in a bath of aqueous alkali. It is an obvious constraint of the plate that the surface must be resistant to damage. The plate must also be dry to the touch. Although this latter constraint has been removed if non-contact imaging as used in CTP is involved, it is a constraint that hitherto has not been appreciated as restrictive in certain areas of performance. In order to achieve dry coatings, the coating formulation is either confined to substances that produce such a dry layer or have added polymers that must be present in quantities that produce the dry layer. The presence of such polymers, and limitations of the kind of photosensitive materials used to produce the dry layers, reduces opportunities to optimize coating sensitivity and simplicity of processing.